1. Field of the Invention
The present invention relates to portable information processors such as notebook personal computers, and more particularly to a portable information processor, a metal housing of the portable information processor, and a method for manufacturing the housing.
2. Background Art
In recent years, portable information processors (hereinafter, collectively referred to as notebook PCs) are becoming increasingly low-profile and lightweight to improve portability. This means that the housings of the notebook PCs are also becoming increasingly low-profile. On the other hand, more and more housings are made of metal to improve their mechanical strength and to effectively radiate heat generated from the electromagnetic shield or inside the notebook PCs.
A conventional notebook PC is described as follows with reference to drawings.
FIG. 4A is an external perspective view of the conventional notebook PC when the display portion is in the closed state. FIG. 4B is a sectional view of the display portion of FIG. 4A when viewed from axis 4B-4B. FIG. 4C is a sectional view of the display portion of FIG. 4A when viewed from axis 4C-4C.
The notebook PC shown in FIGS. 4A-4C includes, between rear housing 24 and front housing 25, a liquid crystal drive circuit (unillustrated) and liquid crystal display panel 26, which are components of a liquid crystal display device.
The conventional portable information processor thus structured is described in detail as follows.
In general, a notebook PC, which is also called a book PC, has a flat box shape with nearly rectangular flat portions when the display portion is in the closed state, so that it can be easily carried in a bag or the like.
Rear housing 24 is made of a metal material such as aluminum to improve the mechanical strength and to effectively radiate heat generated from the electromagnetic shield or inside the notebook PC. Especially in recent years, further weight reduction has been made by replacing aluminum with a magnesium alloy having a smaller specific gravity and a higher strength than aluminum, thereby making the housing as thin as possible.
The reduction of the housing in thickness and weight has allowed notebook PCs to be carried in a bag or the like more often than before. However, in a crowded train or similar situations, a high pressure can be applied from outside to display portion 22 in the bag. This may deform thin rear housing 24 and damage liquid crystal display panel 26 stored inside. To avoid this from happening, as shown in FIG. 4B, rear housing 24 is mechanically reinforced by raising its center portion in the longitudinal direction like an automobile hood while maintaining its basic thickness. This technique is disclosed, for example, in Japanese Patent Unexamined Publications No. H09-062400 and No. 2003-204174.
In the aforementioned conventional structure shown in FIGS. 4A, 4B, and 4C, rear housing 24 is resistant to bending in its width direction (axis 4B-4B) shown by the dotted arrows, but becomes less resistant closer to the center in its longitudinal direction (axis 4C-4C) shown by the solid arrows because of being supported only by its frame and the edge line of the raised portion (within the dotted circle of FIG. 4B).
The following is a description of the procedure of manufacturing the rear housing by press forming.
FIG. 5 shows the method of manufacturing the rear housing by press forming. As shown in FIG. 5, planar material 51 made of a magnesium alloy is placed between blank holder 52 and die set 53 and then press-molded with punch 55 while being heated with heaters 54.
When rear housing 24 of display portion 22 is press-formed to provide a raised portion, the edge line has material distortion, causing the raised portion to have an irregular surface. In the case of being made of a magnesium alloy, planar material 51 is generally press-formed at high temperatures of 200° C. or higher in order to avoid cracks caused by bending the material. During the press forming, the magnesium alloy thermally expands too much to make the surface of the raised portion flat, and the excess portion of the surface makes the surface irregular.
For easier understanding, the sectional view of rear housing 24 is shown in an exaggerated form in FIG. 5; however, the actual level difference between the raised portion and the other portion is only about ±0.5 mm. Even so, such irregularities on the surface of rear housing 24 spoil the appearance. Not only that, the protrusions on the surface can be turned inside out by a slight external force and become depressions, thereby losing the tension of the surface and greatly reducing the strength of the surface.
FIG. 6 shows a process where a rear housing shaped by casting is cooled and shrunk. In the case where rear housing 24 is cast using a casting mold, it shrinks at different rates in different portions while being cooled as shown in FIG. 6. This particularly affects the flat surface of the raised portion of rear housing 24 and makes it irregular.
Similar to FIG. 5, in FIG. 6, the sectional view of rear housing is shown in an exaggerated form for easier understanding; however, the actual level difference between the raised portion and the other portion is only about ±0.5 mm. Even so, such irregularities on the surface of rear housing 24 spoil the appearance. Not only that, the protrusions on the surface can be turned inside out by a slight external force and become depressions, thereby losing the tension of the surface and greatly reducing the strength of the surface.